1. Field of the Invention
This invention relates to surface abrading machines, and more particularly to such surface abrading machines as lapping machines that are used for precision-finishing the surfaces of such works as semi-conductor wafers.
2. Description of the Prior Art
The working principle of lapping machines is to directly transfer the flatness of an upper and a lower lap to the surfaces of the work held therebetween using abrasive particles (lapping material). Accordingly, their working accuracy depends on the following three factors:
(1) Smoothness of the upper and lower laps; PA1 (2) Kind and characteristics (including the method of use) of abrasive particles; and PA1 (3) Difference in the length of orbit (peripheral speed) at different points of the work resulting from the relative motion of the laps and holder.
When the upper and lower laps are smooth enough and abrasive particles of the type optimum for the quality of the work and the required working accuracy are used (i.e., when the aforementioned requirements (1) and (2) for the soft lap surface are satisfied), the composite peripheral speed and the length of orbit at different points of the work, which result from the relative motion of the laps and holder, must be made uniform to improve working accuracy.
In lapping machines of known types, a plurality of holders are engaged with a sun gear at the center and surrounding internal gears in a sun-and-planet fashion, with the works held by the holders being abraded by an upper and a lower lap. Two-way lapping by such lapping machine is performed by rotating the sun and internal gears while stopping the upper and lower laps, thereby causing the holders to rotate on their own axes and revolve around the sun gear like planets. Four-way lapping, on the other hand, is performed by moving the sun and internal gears and the upper and lower lap simultaneously.
In doing lapping, the center of a holder that moves like a planet draws a truly circular orbit around the sun gear, with other points than the center on the holder drawing orbit longer than the one at the center because of the motion associated with the rotation of the holder on its own axis. In other words, the center of the holder draws the shortest orbit, while other points on the holder draw longer orbits, the length of the orbit drawn by each of such other points being proportional to the distance of the point from the center. Consequently, the speed with which the work on the holder is lapped becomes nonuniform in some portions, with a resulting drop in working accuracy.
With this type of lapping machine, the size of laps is such that the ratio of outside diameter to inside diameter is 2.5 to 3 (outside diameter/inside diameter=2.5 to 3). Accordingly, the peripheral speed on the outside is 2.5 to 3 times faster than that on the inside. In four-way lapping, this speed difference produces such an effect as to further drop working accuracy.
When the work demands to be finished with a high degree of accuracy, therefore, it is necessary to reduce the influence of such speed difference to a minimum.